As a process for producing a polytetrafluoroethylene (hereinafter referred to also as “PTFE”) by polymerizing tetrafluoroethylene (hereinafter referred to also as “TFE”) in an aqueous medium, emulsion-polymerization and suspension-polymerization are available.
The emulsion-polymerization is usually carried out by using a fluorinated emulsifying agent together with a stabilizing agent such as a paraffin wax, a fluorine oil or a silicone oil under a mild stirring condition. In such a case, a sufficient amount of the fluorinated emulsifying agent is used so that PTFE particles to be formed would have a particle size of about 0.1 to 0.4 μm and would be stably dispersed in an aqueous medium. After the emulsion-polymerization, particles are coagulated so as to be a wet powder having a particle size of about a few hundreds μm, followed by drying the wet powder to obtain a powder so-called fine powder.
On the other hand, the suspension-polymerization is carried out under a severe stirring condition without using a stabilizing agent such as a paraffin wax, a fluorine oil and a silicone oil and usually without using a fluorinated emulsifying agent. PTFE obtained by the suspension-polymerization is granular polymer particles having mustache shape and a length of a few cm. Thus, such granular polymer particles are washed, followed by milling so as to have a particle size of about 1 to 100 μm to obtain a powder so-called molding powder.
The PTFE thus obtained has an extremely high melt viscosity, is non-melt moldable and thereby cannot be molded by a molding method employed for ordinal thermoplastic resins, for example, extrusion molding or injection molding.
Accordingly, in a case where a PTFE fine powder produced by the emulsion-polymerization is to be molded, a method is employed such that first, the fine powder is mixed with naphtha and preliminarily molded, and then an obtained paste is subjected to extrusion molding to obtain a molded product having a desired shape. The molded product may, for example, be an insulating coating material such as a tube, a hose or an electric cable. Further, by carrying out stretching treatment, a porous film for various applications can be obtained.
On the other hand, in a case where a PTFE molding powder produced by the suspension-polymerization is to be molded, first, the molding powder is packed in a mold at room temperature and compression molded, followed by heating to at least the melting point of the PTFE and sintering so as to obtain a molded product. Then, the molded product is subjected to machine processing such as cutting so as to process into a desired shape. The molded product may, for example, be an industrial member such as a gasket, a lining or an insulating film, or a tank which receives a strong acid or a strong alkali in the semiconductor industry.
As described above, as the polymerization method for producing PTFE, the emulsion-polymerization and the suspension-polymerization are usually employed, and an emulsifying agent is not usually used in the suspension-polymerization, however, a technique is disclosed such that a small amount of an emulsifying agent is used in the suspension-polymerization.
For example, Patent Documents 1 and 2 disclose techniques such that by using a very small amount of a specific fluorinated carboxylic acid type emulsifying agent such as ammonium perfluorooctanoate (hereinafter referred to also as “APFO”), it is possible to increase the specific surface area of granular polymer particles obtained by the suspension-polymerization.
Further, Patent Document 3 discloses a technique such that in the suspension-polymerization, by using a very small amount of a specific fluorinated sulfonic acid type emulsifying agent, it is possible to reduce buildup of granular polymer particles on the inside of a polymerization vessel.